The invention relates to “active implantable medical devices” as defined by the Directive 90/385/EEC of 20 Jun. 1990 of the Council of the European Communities, and particularly to implantable devices that continuously monitor heart rate and if necessary deliver electrical stimulation, resynchronization and/or defibrillation pulses to the heart in case of rhythm disorder detected by the device.
The invention relates especially, but is not limited to, those devices that are in the form of an autonomous capsule intended to be implanted in a heart chamber, including the ventricle.
These capsules are free of any mechanical connection to an implantable (such as a housing of the stimulation pulse generator) or non-implantable (external device such as programmer or monitoring device for patient remote monitoring) main device, and for this reason are called “leadless capsules” to distinguish the capsules from electrodes or sensors disposed at the distal end of a conventional probe (lead), which is traversed throughout its length by one or more conductors galvanically connecting the electrode or sensor to a generator connected to an opposite, proximal end of the lead. A detection/stimulation electrode in contact with the wall of the ventricle enables the capsule to detect the presence or absence of a spontaneous depolarization wave of the cardiac cavity, as well as the occurrence time of the wave (ventricular or atrial marker).
The electrode also allows the delivery of a stimulation pulse in the event of absent or late spontaneous depolarization, so as to cause contraction of the cardiac cavity.
Note, however, that the autonomous nature of the capsule is not inherently a necessary feature of the present invention.
The management of the stimulation energy is a critical aspect of any implantable pacemaker, because it has a direct impact on the power consumption of the integrated pacemaker battery, and thus on its overall lifespan.
This topic is particularly critical in the case of a leadless capsule pacemaker wherein, unlike conventional pacemakers, the energy required for the issuance of stimulation is 70% of the total energy consumed. In addition, it must be considered that the very small dimensions of a leadless capsule imposes minimizing the size of the battery and thus its capacity, as the battery often occupies more than 70% of the volume in a leadless capsule.
In fact, if it was possible to reduce, for example, half the energy required for stimulation, the size of the battery could correlatively be reduced about 40% while keeping the same longevity, which would reduce the volume of the capsule to about 0.6 cm3 (compared to 1 cm3 in the best case today), all performances being equal.
To minimize the energy dedicated to stimulation as much as possible, while maintaining the effectiveness of delivered electrical pulses, a technique called “cycle to cycle capture” may be employed. Cycle to cycle capture maintains the stimulation energy at a minimum level, continuously checking, after each stimulation, if the stimulation was effective (“capture”) or not. If no depolarization wave has been induced by stimulation of the cardiac cavity (“non-capture”), the implant delivers, during the same cardiac cycle, a stimulation of a relatively high energy to ensure the triggering of a depolarization. Then, by successive iterations, the stimulation energy is gradually reduced in each cardiac cycle, so as to converge again to an energy close to the limit or “triggering threshold” needed to cause depolarization of the cardiac cavity.
The invention relates more precisely to a method to determine the pacing threshold by successive approaches, in the most efficient possible method from the energy consumption point of view.
The basic technique which is commonly used today in most pacemakers, is described in U.S. Pat. No. 3,777,762 A. The technique involves using a method of progressive decreases in amplitude (voltage) of the stimulation pulses for a fixed pulse width.
Another technique is described in U.S. Pat. No. 4,979,507 A. This technique relies on the fact that the delivered energy not only depends on the amplitude of the stimulation pulses, but also of the width of these pulses (stimulation duration). The pacing threshold varies as a function of these two parameters according to a nonlinear law called “Lapicque law”.
The technique proposed in U.S. Pat. No. 4,979,507 A includes performing two amplitude scans, with two different pulse widths. This approach has a risk of capture default, because the theoretical Lapicque law defines a boundary between capture and non-capture that, in practice, varies from one patient to another. It is therefore necessary to validate either continuously or at regular intervals the method for each patient, by making a complete scan of all possible values of the parameters (amplitude and width of the stimulation pulse). However, a full scan is impractical because it is very costly in terms of energy and requires interrupting therapy during scanning.
WO 94/12237 Al discloses another technique for automatically adjusting the capture threshold wherein, again, the variation of the energy of one stimulation pulse to the next is made either by changing the duration of the pulse, or by changing the amplitude of the pulse. This significantly increases the number of iterations required for the search algorithm to determine the actual value of the stimulation threshold.
U.S. Pat. Nos. 5,718,720 A, 5,702,427 A, 5,549,652 A and 6,650,940 B1 describe other techniques for determining the pacing threshold, implementing various capture detection methods such as a direct detection of mechanical myocardial contraction, analysis of an accelerometric signal, analysis of a temperature signal, analysis of intracardiac pressure, etc.